ECG Intervals & Waves
Master the key ECG intervals and waves — normal ranges, measurement techniques, and clinical significance of abnormalities.
PR Interval
The PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization. It reflects conduction through the atria, AV node, Bundle of His, and proximal bundle branches.
0.12–0.20 seconds (120–200 ms) in adults at normal heart rates. Values below 0.12s suggest pre-excitation or accelerated AV conduction; values above 0.20s indicate first-degree AV block.QRS Complex
The QRS complex represents rapid ventricular depolarization as the electrical impulse spreads from the Bundle of His through the right and left bundle branches and Purkinje fiber network to the ventricular myocardium. Its duration reflects the speed and synchrony of this depolarization.
Less than 0.12 seconds (120 ms) in adults. Values of 0.10–0.12s are considered borderline. A QRS duration of 0.12 seconds or greater indicates a bundle branch block or other intraventricular conduction delay.QT Interval
The QT interval represents the total duration of ventricular depolarization and repolarization, from the onset of the QRS complex to the end of the T wave. Because QT duration varies with heart rate, it is routinely corrected to a standardized rate of 60 bpm to produce the QTc (corrected QT interval).
QTc less than 450 ms in males, less than 460 ms in females. Values of 450–500 ms (males) or 460–500 ms (females) are borderline prolonged. QTc greater than 500 ms carries a substantially increased risk of Torsades de Pointes and sudden cardiac death. Normal QT without rate correction is approximately 0.36–0.44 seconds at 60 bpm.ST Segment
The ST segment is the flat, isoelectric portion of the ECG between the end of the QRS complex (J point) and the beginning of the T wave. It represents the plateau phase of the ventricular action potential (phase 2), during which the myocardium is uniformly depolarized and there is minimal net current flow. Deviations from the isoelectric baseline are among the most clinically important findings in electrocardiography.
The ST segment is normally isoelectric (at the baseline), with no more than 1 mm (0.1 mV) of elevation or depression in limb leads and no more than 1–2 mm in precordial leads (with early repolarization variants allowed up to 2 mm in some guidelines). The J point may be slightly elevated in early repolarization, particularly in young males.P Wave
The P wave represents atrial depolarization — the spread of an electrical impulse from the sinoatrial (SA) node through both atria. The first half of the P wave reflects right atrial activation, while the second half reflects left atrial activation. P wave morphology, duration, and axis provide important information about SA node function, atrial size, and the site of impulse origin.
Normal P wave duration is less than 0.12 seconds (120 ms). Normal P wave amplitude is less than 2.5 mm in limb leads and less than 1.5 mm in V1. The P wave is normally upright (positive) in leads I, II, aVF, and V4-V6, and inverted (negative) in aVR. A normal P wave axis is 0 to +75 degrees.T Wave
The T wave represents ventricular repolarization — the recovery phase during which ventricular myocardial cells restore their resting membrane potential after depolarization. Because repolarization occurs in the opposite direction from depolarization (epicardium to endocardium), the T wave is normally in the same general direction (concordant) as the dominant QRS deflection in most leads.
The T wave is normally upright in leads I, II, and V3-V6, and inverted in aVR. T waves in V1 and aVL can be upright or inverted normally. T wave amplitude is normally less than 5 mm in limb leads and less than 10 mm in precordial leads. In V1-V3 of women and children, T wave inversion is a normal finding. T waves are concordant with the QRS: upright where the QRS is predominantly positive, and inverted where the QRS is predominantly negative.